In the case of osteosynthetic treatments of fractures or the implantation of artificial joints, implants, such as bone plates or implant parts, are frequently fixed in a position at the bone by means of bone screws. Such bone screws frequently are self-drilling and self-cutting, and are screwed with a drill or a similar device into the bone. In so doing, the maximum torque, which is exerted on the bone screw and, accordingly, on the threaded connection between the bone screw and the bone, is to be limited to a particular value.
A device for limiting the maximum torque, which can be applied on a screw during a surgical intervention, is described in U.S. Pat. No. 6,132,435 to YOUNG. This known device comprises a driving shaft, which contains a disk-shaped segment with boreholes, disposed on a circle concentric with the axis of rotation, for accommodating balls, a driving shaft segment, which also contains a disk-shaped segment with spherical-shaped depressions, disposed complementary to the boreholes, and a housing, in which the driving shaft and the driving shaft segment are mounted. The balls are pressed by means of springs into the depressions in the disk-shaped segment of the driving shaft segment, the force of the springs being adjustable by means of loosening or tightening a threaded connection between the disk-shaped segment at the driving shaft and the housing.
It is a disadvantage of this known device that the same maximum torque is transferred in both directions of rotation. This is disadvantageous in surgical technique if a bone screw has become ingrown and, as a result, the torque for loosening it during removal has increased.
The invention is to provide a remedy here. It is an object of the invention to provide a device, which permits the transfer of a maximum torque, once set, only in a desired direction of rotation, while in the other direction of rotation, for example, for loosening screws, the torque, which can be transferred, may be larger or smaller.